This invention relates to a method for recovering petroleum from a subterranean, petroleum-containing formation. Specifically, it involves injecting a solution comprising an oligomeric surfactant into the formation. We have found that this method results in a higher salt and divalent ion tolerance, higher solubilization capacity, lower interfacial tension with oil and brine, lower adsorption loss, and higher viscosity.
Petroleum is found in subterranean formations or reservoirs in which it has accumulated, and recovery is accomplished initially by penetrating the reservoirs with one or more wells and pumping or permitting the petroleum to flow to the surface of the earth through these wells. Petroleum can be recovered from subterranean petroleum-containing formations only if certain conditions are present. For example, there must be an adequately high concentration of petroleum in the formation, and there must be sufficient permeability or interconnected flow channels throughout the formation to permit the flow of fluids therethrough if sufficient pressure is applied to the fluid.
When the subterranean petroleum-containing formation has natural energy present in the form of underlying active water drive, solution gas, or a high pressure gas cap above the petroleum within the formation, this natural energy is utilized intially to recover petroleum. This initial phase of petroleum recovery is referred to as primary recovery. When this natural energy source is depleted, or in the instance of those formations which do not originally contain sufficient natural energy to permit primary recovery operations, some form of supplemental recovery process must be utilized. Supplemental oil recovery is frequently referred to as secondary recovery, although in fact it may be primary, secondary, or tertiary in sequence of employment.
The most commonly utilized and economical form of supplemental recovery is water flooding, which involves the injection of water into the formation to supply the energy necessary to displace oil toward producing wells, where it is transported to the surface. Although it is generally possible to recover some additional oil by means of water flooding, from 30 to 60 percent of the oil originally in place in the formation still remains in the formation at the conclusion of a conventional water flooding operation. The failure of water flooding to recovery this remaining oil stems from a plurality of problems discussed below, which have a cumulative effect on the overall oil recovery program.
When water is forced through a capillary such as the flow channels present in subterranean petroleum-containing formations, said flow channel containing both oil and water, the efficiency with which the injected water displaces the petroleum in the formation is relatively low. This inefficient capillary displacement occurs because water and oil are immiscible and the interfacial tension between water and oil is quite high. Persons skilled in the art of oil recovery have recognized this inherent weakness of water flooding, and additives have been described in the prior art for decreasing the interfacial tension between the injected water and the formation petroleum. For example, U.S. Pat. No. 2,233,381 (1941) disclosed the use of polyglycolethers as a surface active agent to increase the capillary displacement efficiency of an aqueous flooding medium. U.S. Pat. No. 3,302,713 discloses the use of a petroleum sulfonate type of anionic surfactant, prepared from the 850.degree. to 1,050.degree. F. boiling range fraction of petroleum for use in an oil recovery operation. U.S. Pat. No. 3,468,377 (1969) describes the use of petroleum sulfonates of specified molecular weight for oil recovery. Other surfactants which have been proposed for surfactant oil recovery include alkyl polyalkoxylates, alkyl sulfates, alkyl sulfonates, and alkyl polyalkoxylated sulfates or sulfonates.
One major problem associated with the use of petroleum sulfonates or other alkylsulfonates and alkylaryl sulfonates is that these surfactants have little tolerance to high concentration of salts. They lose their surface activity in such environments. On the other hand, the polyalkoxylated surfactants, ionic as well as nonionic types, are known to have much higher salt tolerance but lower surface activity. Thus, it has been reported in U.S. Pat. No. 3,508,612 that a two-component surfactant mixture exemplified by a petroleum sulfonate and a polyalkoxylated surfactant exhibits improved tolerance against high salinity environments. This type of formulation has a serious drawback in that, while the salt tolerance is improved, other properties vital to the oil recovery process are sacrificed. The most often encountered problems are that the interfacial tension with oil becomes higher, the solubilization capacity of the formulation drops, the adsorption of surfactants on rock surfaces increases, and the viscosity of the formulation decreases.
Another serious concern in using a mixed formulation of petroleum sulfonates (or other surfactants operative at low salinities) and polyalkoxylated surfactants is that they tend to separate from each other when injected into the reservoir, since they have quite different characteristics in their interactions with reservoir oil, brine, and rock surfaces while they do not have sufficient mutual interaction to keep them together. Further, all commercial polyalkoxylated surfactants have an extremely broad distribution of molecular weights; they themselves tend to separate from each other in the formation. Since the formulation is optimized or designed as a whole for the reservoir, the break down of original composition in the formulation greatly reduces their oil recovery efficiency.
The present invention relates to a process and a composition used in surfactant flooding process, which alleviate above problems. In accordance with this invention, a process is provided for recovering oil from a subterranean formation wherein fluid containing an oligomeric surfactant or surfactant mixture is injected into the formation. The surfactant is characterized by the general formula: EQU R--X--(CH.sub.2 --CR.sub.1 Y).sub.a --(CH.sub.2 --CR.sub.2 Z).sub.b --H,
where X, Y, and Z are polar or ionic functional groups; R, R.sub.1 and R.sub.2 are hydrocarbon chains; and a and b are average numbers of repeating units.
We found that by using a formulation comprising an oligomeric surfactant, with or without other surfactants or cosolvents, in conditions commonly found in oil fields, we can achieve enhanced oil recovery. Accordingly, it is the primary object of this invention to describe such a process.
Another more specific objective of this invention is to describe the use of oligomeric surfactant as a cosurfactant in a formulation for oil recovery processes, which results in a higher salt and divalent ion tolerance, higher solubilization capacity, lower interfacial tension with oil and brine, lower absorption loss, and higher viscosity than the original formulation. A process of using oligomeric surfactants at low concentration to improve waterflood is also described.